Periodic acid
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Periodic acid is HIO4 or H5IO6. The name is not derived from "period", but from "iodine": per-iodic acid (compare iodic acid, perchloric acid), and it should thus be pronounced per-iodic and not as in the usual meaning of periodic.
In dilute solution, periodic acid exists as H+ and IO4−. When more concentrated, orthoperiodic acid, H5IO6, is formed. This can be obtained crystalline. Orthoperiodic acid can be dehydrated to metaperiodic acid, HIO4. Further heating gives diiodine pentoxide (I2O5) and oxygen; apparently the anhydride 'diiodine heptoxide' does not exist in nature but can be formed synthetically.
Thus, two forms of periodates exist. One relating to the acid HIO4, the other relating to H5IO6. The former results in metaperiodates (meta- meaning less water) and the latter, orthoperiodates (ortho- meaning more water). Metaperiodates have solubilities and chemical properties similar to perchlorates (similar but larger ion size) though they are less oxidizing than perchlorates.
Periodic acid is also used in organic chemistry for structural analysis. Periodic acid will cleave a vicinal diol into two aldehyde fragments. This can be useful in determining the structure of carbohydrates.
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